Mike Everman wrote:
Maybe we should have a little contest between the hard core builders here, for "best" (whatever THAT means) implementation of a straight piper at 22" long.

BTW Mike and Larry:

I'd like to see the allowable length increased to 32 inches. The commonly-used 1-inch conduit (for intake, etc.) implies a combustion chamber diameter of something like 2 inches. Using the available data for combustion duration, that means the 1/frequency to support the necessary burn time implies a length of 32 inches. The "Laird Chinese" supports this contention, BTW.

WE CAN use the shorter length--but the combustion event will be severely compromised, and the SFC will suffer accordingly.

Bill H.
Acoustic Propulsion Concepts

".......some day soon we'll be flying airplanes powered by pulsejets."

Bill, I think that picture is a bullet shrouded bent linear. I think it was M. that knew something about that fella.

As to the contest, I only mentioned "straight piper of 22"" because Larry's so into the Dynajet form factor and that happens to be what he's building right now, so I've got one guaranteed taker, ha. ;-) Ideally for the contest it'd be a 2.5" CC, too. THAT is a tall order for a valveless on gas or propane! (no acetylene allowed!)

The allure to me is exactly the problem Bill mentions, the need to accelerate burn time for the shorter form to make sense, if it ever will. That would be one of the innovations that could, maybe, perhaps pop out the other end of the contest, shorter ducts with larger diameters; bigger gas exchanges at higher frequency. The guy that can build that WITH tail expansion will be a PJ God to my way of thinking!

I built a 24" straight linear piper, and it was verrrry hard to start, and was sensitive to intake length like none I've built.

We could do 32" but it'll be a Chinese contest. Just trying to explain my motivations for shorter.

Jon, no reason for equal exits other than it's known to work on the Chinese (Laird) and the Escopette on which it was based. Smaller intake diameters and straight tails are the easiest to get going, until you make them exceedingly short, then the high frequency rushes things for propane burn rate. Additionally, once you go for tail expansion, it becomes more finicky about where diameter transitions happen.

As your motor becomes more "front loaded", that is, the intake is bigger in diameter, it becomes much more finicky about over-all length (as you try for shorter motors). I've noticed from working motors that there become two solutions for the intake length, short and long, and each length has to make sense acoustically with the whole. A few solutions for intake in this case hover around 3.75:1, 4.25:1 and 7.5:1 for your aspect ratio (L/d), with the shorter one very hard to pull off on a short motor.

Rear loaded or equal exits from the CC allow your starting point on intake aspect ratio to be around 6.25:1.

When Kentfield went to 4 intake tubes on his (front loaded) motor that had the same total intake area as a single inlet SNECMA type, an interesting thing happened. The four tubes were of course half the diameter of the single, and the aspect ratio of the intake tube(s) were then able to go from 4.5 to 6.25:1, seemingly because each by itself was able to use the rear loaded intake ratio. Faaascinating.

We can talk about how the intake length is then relates to the "length of all else" in a bit; I have to go to work!

I only changed one thing. I did not notch out the CC. I just stuffed the exhaust pipe 3.74 inches into the CC and welded it. I jamed a piece of wood while welding to make sure that it was straight. Hopefully if she runs it won't sag.

The only thing else I that I did was flared out the exhaust tube inside the CC on the intake side only. I was hoping that this would create more turbulance on the intake pulse and move the air towards the upper corner of the CC.

I have not yet flared the exhaust end just incase I need to extend the length.

hagent wrote:I only changed one thing. I did not notch out the CC. I just stuffed the exhaust pipe 3.74 inches into the CC and welded it. I jamed a piece of wood while welding to make sure that it was straight. Hopefully if she runs it won't sag.

Blast ... I come up with the simplest thing in the world, and someone proves it can be simplified ;-)

The only thing else I that I did was flared out the exhaust tube inside the CC on the intake side only. I was hoping that this would create more turbulance on the intake pulse and move the air towards the upper corner of the CC.

That should be OK. Eric would disapprove, of course, having come to the conclusion that trying the "shape" the intake flow is counterproductive. But, I think you should be OK with that as long as your flare is not large.

I have not yet flared the exhaust end just incase I need to extend the length.

Good plan - one can never really be sure before firing it. I have often said that the Hinote Criteria should be used as a place to start, not the final word on valveless design. It never did work for me on the Elektras - but those were very oddly shaped chambers.

I do have to say that it didn't take very long to make this.

Well, of course not. I doubt very much that the tailpipe will sag, with that 3/4 circumference weld. Good luck on getting her running. You should be able to go very light on the starting air to get her going. Or, make yourself one of my combined air/fuel pipes (probably the best idea I've ever published, but NO ONE ever tries it but me!)

hagent wrote:
I only changed one thing. I did not notch out the CC. I just stuffed the exhaust pipe 3.74 inches into the CC and welded it. I jamed a piece of wood while welding to make sure that it was straight. Hopefully if she runs it won't sag.

Cool -- that's essentially what I was proposing on the previous page -- the only difference is that you have the intake on one side of the ellipse instead of being in the center.

Larry,
I bet there companies that sell blowpipe/fuel gizmos for some purpose or another. When you think about it, there are a lot of variations on a theme. A common car carburetor, the metering jet inside the throat of a Dynajet, a propane torch nozzle, a Bunsen Burner, or for that matter an acetylene/oxygen torch in a way all do the same thing if you stretch the evolution of fuel/air mixing.
There are hardly any new ideas that have come about, perhaps adaptations on a theme would be more correct.
I hadn't seen any posts of yours lately, I was wondering if it was gettting to cold to experiment "up there." Last night I had to turn my air conditioner on, it was unseasonably warm and 97% humidity which didn't help my 2.5 gallon tank snorkeling testing.
Mark

I can admittedly say that the NRL starting assembly is 'prior art' for my fuel/air mixer, though I wasn't really thinking about that engine at the time. My only real contribution is to show that you can do it with simple hand methods and without machining - AND prove (to myself, at least) that it really works, providing quick, sure starts every time. The real benefit to this scheme is that is gives you a truly consistent starting method - almost all the variables of getting the starting air "just right" have been eliminated.

The starting tube idea works so well that I'm sure that if you provided electric switches that simultaneously activated spark, fuel and air, you could actually produce instant acting roarers (pipes a little too short to sustain) and build yourself a "pulsejet organ" that could be played from normal manual and pedal keyboards.

Of course, my design still seems intricate to some observers. Not everyone wants to do blacksmithing on a near-microscopic level. But, there is another way. Just flatten the upper straight end of the brake line about halfway (forming a velocity nozzle) and lash the copper tube flush against it with fine wire, and maybe solder together for good measure. In other words, make it "almost concentric" rather than truly concentric. As long as you get the ends of both pipes properly aimed into (and centered in) the intake tube, this should be every bit as good, giving the same perfectly consistent performance.

I wholely agree Larry, your blowpipe device is akin to what I think about often when pulsejetting. Let's say you've got a weak spark unit and you are testing in hot humid air with methanol which has, after a few false starts coated the inside of your jet with water vapor further compounding the problem. Or maybe your spark unit is cutting in and out with a bad connection or shocking you from time to time from the less than idea setup.
My point is, the more you can iron out all the little problems, the easier it is to get on with the discovery part. Your blowpipe air would be a blessing to some if they just realized it's a little work at first but will pay for itself down the line.
Mark

We found at Burning Grape that the joint where fuel tube met air tube to be excessively leaky, but maybe we were only concerned because compressed air became precious. Some solder there would go a long way.

On the two tube motor: I have great doubts that it will work, there is no restriction aft of the CC. I suspect it will be a torch only, unless you can make a restriction.